Spermatogenesis occurs in a highly organized tissue, the seminiferous epithelium which contains proliferating germ cells and a static population of somatic cells. The germ cells exist in defined cellular associations and are intimately associated with the nongerminal, Sertoli cells. There is substantial evidence that Sertoli cells are important components in the regulation of spermatogenesis. Furthermore, the morphology and physiology of the Sertoli cells change in consort with the development of their neighboring germ cells. It seems likely that the germ and Sertoli cells interact with each other and that this interaction is important in the regulation of spermatogenesis. This cellular interaction may be facilitated by the secretion and binding of specific regulatory proteins. Consequently, it is proposed to examine a newly discovered protein, Cyclic Protein-2, which is secreted maximally by seminiferous tubules at States VI-VIIa, b of the cycle of the seminiferous epithelium. In particular, it is proposed: first, to isolate this protein and raise a monospecific antiserum; second, to use this antiserum to identify the cellular orgin of CP-2; third, ot identify the cellular target of CP-2 and biochemically characterize the binding of CP-2 to that cell and finally; to examine changes in the secretion and binding of this protein with testicular maturation and with progression of the epithelial cycle in the mature animal.